Sorbent system for removing ammonia and organic compounds from a gaseous environment

ABSTRACT

A first process and sorbent for removing ammonia from a gaseous environment, the sorbent comprised of graphene oxide having supported thereon at least one compound selected from metal salts, metal oxides and acids, each of which is capable of adsorbing ammonia. A second process and sorbent system for removing ammonia and a volatile organic compound from a gaseous environment; the sorbent system comprised of two graphene-based materials: (a) the aforementioned graphene oxide, and (b) a nitrogen and oxygen-functionalized graphene. The sorbents are regenerable under a pressure gradient with little or no application of heat. The processes are operable through multiple adsorption-desorption cycles and are applicable to purifying and revitalizing air contaminated with ammonia and organic compounds as may be found in spacesuits, aerospace cabins, underwater vehicles, and other confined-entry environments.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.16/008,115, filed Jun. 14, 2018, now allowed, which claims the benefitof U.S. provisional application No. 62/530,327, filed Jul. 10, 2017, theapplications in their entirety incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with support from the U.S. Government underContract No. NNX16CJ41P, awarded by the National Aeronautics and SpaceAdministration. The Government has certain rights in this invention.

FIELD OF THE INVENTION

In one aspect, this invention pertains to a first process and sorbentfor removing ammonia from a gaseous environment. In another aspect, thisinvention pertains to a second process and sorbent system for removingammonia and a volatile organic compound (VOC), such as formaldehyde,from a gaseous environment. The aforementioned processes employ one ormore graphene-based sorbents.

BACKGROUND OF THE INVENTION

Trace contaminants introduced into a ventilation loop of an enclosed orentry-limited environment, such as a spacesuit or a vehicular cabin, aretypically removed via sorption with activated carbon. Such tracecontaminants predominantly include ammonia and formaldehyde. Bothammonia and formaldehyde as well as other contaminants are produced viametabolic processes, or off-gassing of construction and spacesuitmaterials, or are produced as by-products of an amine used in carbondioxide sorbent systems, such as, the Rapid Cycle Amine (RCA) systemdeveloped by United Technologies Corporation as sponsored by NASA. Basedon studies performed by NASA, a Trace Contaminant Control (TCC) deviceis required in a ventilation loop of a Portable Life Support System(PLSS) of a spacesuit in order to control ammonia and formaldehyde fromexceeding their Spacecraft Maximum Allowable Concentration (SMAC)levels. On a larger scale, ammonia occurs as a waste gas in industrialprocesses and in animal husbandry environments, the latter exemplifiedby the poultry industry. In industrial processes, scrubbers comprisingactivated carbon typically are used for ammonia removal. In animalhusbandry, ventilation with outside air is employed requiring expensiveconditioning via heating or cooling.

Although activated carbon is effective as a sorbent for ammonia andVOC's, activated carbon comes with well-recognized disadvantages.Activated carbon provides a bulky, weighty sorbent system. Moreover, asorbent bed comprising powdered activated carbon has a high pressuredrop that is subject to channeling and non-uniform flow distribution. Inaddition, activated carbon suffers from low regeneration capability andunacceptable power consumption. Specifically, activated carbon sorbentsrequire an elevated temperature with extended power consumption forthermal regeneration. More disadvantageously, the activated carbonsorbent of NASA's spacesuit TCC system cannot be regenerated during anExtravehicular Activity (EVA) mission where the elevated temperature isunavailable. EVA regeneration is also prohibited for its unacceptablepower consumption as well as its flammability hazard on the mission'sessentially pure oxygen environment.

In the way of further disadvantages, where the sorbent must removemultiple contaminants from the same gaseous environment, activatedcarbon suffers from competitive adsorption wherein more stronglyadsorbed compounds replace and outgas weakly adsorbed compounds. Duringthermal regeneration, activated carbon is also prone to loss of sorbentcapacity and mass through oxidation and attrition, respectively. Thermalregeneration of activated carbon also poses a fire hazard. Lastly, asorbent based on activated carbon relies on a consumable, which meansthe activated carbon requires replacement at the end of its life cycle.In view of the above, life cycle operating costs of employing activatedcarbon as a sorbent remain unacceptable for regenerative contaminantcontrol.

Molecular sieves and zeolites are known sorbents and well researchedinto related effects of pellet size and shape, humidity level, and flowrate on regenerative contaminant control. Pelletized sorbents likemolecular sieves, zeolites, and even activated carbon, exhibit anadsorption capacity limited by diffusion of sorbate into the pelletcore, thereby restricting access to available adsorption capacity.Pellets also suffer from too high a pressure drop and flow channeling.Sorbent coatings on monolithic supports offer high geometric surfacearea per unit volume and reduced pressure drop, but they are limited byloading capacity. Sorbents based on molecular sieves and zeolites alsorequire thermal-assisted regeneration, that is, an addition of heat tobe fully regenerated, which as noted earlier is unfeasible either forits unacceptable power consumption or its lack of availability, or both.Moreover, zeolites and molecular sieves have reduced cyclic capacitywhen operating under vacuum swing regeneration and when exposed to humidair.

The art would benefit from discovery of an improved process and sorbentfor removing contaminants from a gaseous environment, more particularly,for removing trace contaminants from a gaseous environment, mostnotably, for removing ammonia and optionally a volatile organiccompound, such as formaldehyde, from a gaseous environment. Such asorbent should desirably provide improved sorbent capacity towards thecontaminants of interest as well as good thermal and attritionresistance. Desirably, the sorbent should be capable of regeneration ata temperature less than about 40° C. so as to minimize powerconsumption. Even more desirably, the sorbent should offer lighterweight, smaller volume, lower pressure drop, a uniform flow distributionessentially without channeling, and a lower reliance on consumables, ascompared with present day sorbents.

SUMMARY OF THE INVENTION

In a first aspect, this invention provides for a novel sorbentcomposition comprising graphene oxide having supported thereon at leastone compound selected from metal salts, metal oxides, and acids, each ofwhich is capable of reversibly adsorbing ammonia.

In a related first aspect, this invention provides for a novel processof removing ammonia from a gaseous environment, comprising contacting agaseous feedstream comprising an initial concentration of ammonia with asorbent under conditions sufficient to produce an effluent streamcomprising a reduced concentration of ammonia as compared with theinitial concentration. In this related first aspect, the sorbentcomprises the graphene oxide having supported thereon at least onecompound selected from metal salts, metal oxides, and acids, each ofwhich is capable of reversibly adsorbing ammonia.

In these aforementioned first aspects, we have discovered a novelsorbent useful for removing ammonia from any gaseous environmentcomprising ammonia as a contaminant or undesirable component including,without limitation, gaseous environments wherein ammonia occurs inincreasingly unacceptable concentrations as a byproduct of metabolicprocesses or outgassing of materials, or as a result of amine sorptionof carbon dioxide, or as a waste product of an industrial process or awaste product in an animal husbandry environment. As the sorbent, ourprocess employs a high surface area nanomaterial, specifically, grapheneoxide having supported thereon one or more compounds selected from metalsalts, metal oxides and acids, which each one and all are capable ofreversibly adsorbing ammonia. As used herein and throughout thisdescription, the term “reversibly adsorbing” generally means that asorbent is capable of binding to its surface a sorbate, such as ammoniaor a volatile organic compound depending upon the selected sorbent, upto a saturation or partial saturation level thereby removing the sorbatefrom a gaseous environment; and thereafter the saturated or partiallysaturated sorbent is capable of releasing the bound sorbate underregeneration conditions so as to produce the sorbent with regeneratedsorbent capacity.

In a second aspect, this invention pertains to a sorbent systemcomprising the following components:

-   -   (a) a graphene oxide having supported thereon at least one        compound selected from metal salts, metal oxides, and acids,        each of which is capable of reversibly adsorbing ammonia; and    -   (b) a nitrogen and oxygen-functionalized graphene prepared by a        process comprising, contacting graphene oxide with an amine of        the formula NHR₂, wherein each R is independently selected from        the group consisting of hydrogen, C₁₋₇ alkyl, and C₁₋₇        aminoalkyl, the contacting occurring under reaction conditions        sufficient to produce the nitrogen- and oxygen-functionalized        graphene.

In a related second aspect, this invention provides for a novel processof removing a plurality of contaminants including ammonia and a volatileorganic compound from a gaseous feedstream. The novel process of thisinvention comprises contacting a gaseous feedstream comprising aninitial concentration of ammonia and an initial concentration of avolatile organic compound with a sorbent system under conditionssufficient to produce an effluent stream comprising reducedconcentrations of ammonia and the volatile organic compound as comparedwith their respective initial concentrations. In this related secondaspect, the sorbent system comprises the following components:

-   -   (a) a graphene oxide having supported thereon at least one        compound selected from metal salts, metal oxides, and acids,        each of which is capable of reversibly adsorbing ammonia; and    -   (b) a nitrogen and oxygen-functionalized graphene prepared by a        process comprising, contacting graphene oxide with an amine of        the formula NHR₂, wherein each R is independently selected from        the group consisting of hydrogen, C₁₋₇ alkyl, and C₁₋₇        aminoalkyl, the contacting occurring under reaction conditions        sufficient to produce the nitrogen- and oxygen-functionalized        graphene.

In these second aspects of this invention, the process employs a novelsorbent system comprising two graphene-based nano materials, onecharacterized by a high surface area and sorbent capacity for ammonia,and the other characterized by a high surface area and sorbent capacityfor volatile organic compounds. This novel sorbent system beneficiallyexhibits improved regenerability at a temperature less than 40° C.,resulting over time in reduced power consumption, preserved sorbentcapacities, less thermal degradation, less attrition, and improved lifecycle of the individual sorbents. In a preferred embodiment, the sorbentsystem of this invention is supported on a mesh support, which providesfor lower weight and lower volume, a reduced pressure drop, and improveduniformity of flow distribution as compared to other support systems,such as pellets, as well as a lessened reliance on consumables ascompared with activated carbon.

The graphene-based sorbents of this invention advantageously exhibit anexcellent degree of regenerability after the sorbent is saturated orpartially saturated with contaminant(s). Regeneration is easilyaccomplished by flushing the saturated or partially saturated sorbentwith dry uncontaminated air or, alternatively, by pressure (vacuum)swing regeneration with little or no application of heat. By eliminatingelevated temperatures, the regeneration procedures of this inventionadvantageously result in a safer process with power savings. Moreover,sorbent regeneration as provided by this invention can be performedduring EVA missions.

Tests performed at higher humidity, for example, 40 percent or higherrelative humidity, on the graphene-based sorbents of this inventionshowed little effect of water on the retention capacities for ammoniaand the volatile organic compound, which provides another beneficialfeature allowing for flexibility in operation. As a further advantage,the sorbents of this invention exhibit little, if any, effect inperformance on exposure to carbon dioxide, a ubiquitous component inenclosed spacesuit or cabin environments as well as in waste combustionstreams. The graphene sorbents of this invention provide superiorperformance due to their relatively high surface area and specificchemistry, with added potential for low manufacturing costs inlarge-scale.

One utility for the processes and sorbent compositions of this inventioninvolves removing two significant trace contaminants, namely ammonia andformaldehyde, from the gaseous environment circulating throughspacesuits, such as in NASA's TCC system attached to its Portable LifeSupport System. Other utilities for the processes and sorbentcompositions of this invention include air revitalization andpurification in passenger aircraft, atmosphere revitalization inunderwater vehicles, air purification in ground crew cabins and groundvehicles, as well as cleaning indoor air in enclosed or confined-entrysystems. Our sorbents are also envisioned to be adaptable to present dayheating, ventilation, and air conditioning systems (HVAC systems) inconjunction with existing particle filters to directly clean indoor airwhile decreasing a requirement for costly outside air ventilation.Currently, about 20 to 40 percent of energy consumed in U.S. commercialand residential buildings is used for HVAC. In another embodiment, theprocesses and sorbent compositions of this invention are employable inpurifying air in stand-alone room purification units, for example, as adrop-in replacement for activated carbon. Another utility for theprocesses and compositions of this invention involves scrubbing wastegas streams from industrial processes as well as from animal husbandryenvironments, poultry farms exemplifying an important application. Thisapplication envisions drop-replacing conventional activated carbonscrubbers with the novel sorbent(s) of this invention for increasedsorbent capacity and sorbent lifetime.

DRAWINGS

FIG. 1 depicts a graph plotting ammonia outlet concentration as afunction of time in an embodiment of this invention in which ammonia isremoved from a gaseous feedstream using the ammonia sorbent in powderform.

FIG. 2 illustrates a graph plotting ammonia outlet concentration as afunction of time in an embodiment of this invention in which ammonia isremoved from a gaseous feedstream using the ammonia sorbent supported ona metal mesh.

FIG. 3 illustrates a graph plotting ammonia outlet concentration as afunction of time in an embodiment of this invention in which ammonia andformaldehyde are removed from a gaseous feedstream using the dualgraphene-based sorbent system of this invention.

DETAILED DESCRIPTION OF THE INVENTION

In one exemplary embodiment of this invention, there is provided a novelsorbent composition comprising graphene oxide having supported thereonat least one compound selected from metal salts, metal oxides, andacids, each of which is capable of reversibly adsorbing ammonia, whereinsaid graphene oxide is itself supported on a mesh support having anultra-short-channel-length.

In another exemplary embodiment of this invention, there is provided anovel process of removing ammonia from a gaseous environment, comprisingcontacting a gaseous feedstream comprising an initial concentration ofammonia with a sorbent under conditions sufficient to produce aneffluent stream comprising a reduced concentration of ammonia ascompared with the initial concentration. In this exemplary embodiment,the sorbent comprises the aforementioned graphene oxide having supportedthereon at least one compound selected from metal salts, metal oxides,and acids, each of which is capable of reversibly adsorbing ammonia,wherein said graphene oxide is itself supported on a mesh support havingan ultra-short-channel-length.

In another exemplary embodiment of this invention, there is provided anovel sorbent system comprising the following components:

-   -   (a) a graphene oxide having supported thereon at least one        compound selected from metal salts, metal oxides, and acids,        each of which is capable of reversibly adsorbing ammonia; and    -   (b) a nitrogen and oxygen-functionalized graphene prepared by a        process comprising, contacting graphene oxide with an amine of        the formula NHR₂, wherein each R is independently selected from        the group consisting of hydrogen, C₁₋₇ alkyl, and C₁₋₇        aminoalkyl, the contacting occurring under reaction conditions        sufficient to produce the nitrogen- and oxygen-functionalized        graphene;        wherein said graphene oxide and said functionalized graphene are        each supported on a mesh support having an        ultra-short-channel-length.

In yet another exemplary embodiment of this invention, there is provideda novel process of removing ammonia and a volatile organic compound froma gaseous environment, comprising contacting a gaseous feedstreamcomprising an initial concentration of ammonia and an initialconcentration of a volatile organic compound with a sorbent system underconditions sufficient to produce an effluent stream comprising reducedconcentrations of ammonia and the volatile organic compound as comparedwith their respective initial concentrations; wherein the sorbent systemcomprises the following components:

-   -   (a) a graphene oxide having supported thereon at least one        compound selected from metal salts, metal oxides, and acids,        each of which is capable of reversibly adsorbing ammonia; and    -   (b) a nitrogen and oxygen-functionalized graphene prepared by a        process comprising, contacting graphene oxide with an amine of        the formula NHR₂, wherein each R is independently selected from        the group consisting of hydrogen, C₁₋₇ alkyl, and C₁₋₇        aminoalkyl, the contacting occurring under reaction conditions        sufficient to produce the nitrogen- and oxygen-functionalized        graphene;        wherein said graphene oxide and said functionalized graphene are        each supported on a mesh support having an        ultra-short-channel-length.

The gaseous environment from which the gaseous feedstream is derivedencompasses any gaseous mixture comprising ammonia (NH₃) and at leastone other gaseous component preferably selected from nitrogen, oxygen,air, carbon monoxide, carbon dioxide, water, and mixtures thereof. Inone embodiment, the gaseous feedstream additionally comprises at leastone volatile organic compound (VOC), which include any compoundcomprising at least carbon and hydrogen atoms and having a measureablevapor pressure. The concentrations of ammonia and any VOC in the gaseousfeedstream depend upon the source of the gaseous environment and eachcomponent's partial pressure therein. Generally, ammonia and any VOCpresent in the feedstream are each independently present in aconcentration ranging from several parts per billion by volume (ppb_(v))to many thousands of parts per million by volume (ppm_(v)). In oneembodiment, ammonia is present as a contaminant in the feedstream in aconcentration ranging from greater than about 0.1 ppm_(v) (100 ppb_(v))to less than about 10,000 ppm_(v). In another embodiment, ammonia ispresent in the feedstream in a concentration ranging from about 10ppm_(v) to about 100 ppm_(v). It is noted that the permissible SMAC ofammonia during an EVA mission is only 20 ppm_(v). In another embodiment,each VOC, if present, is present as a contaminant in the feedstream in aconcentration ranging from about 0.1 ppm_(v) to about 10,000 ppm_(v). Inanother embodiment, formaldehyde, if present, is present as acontaminant in the feedstream in a concentration ranging from about 0.1ppm_(v) to about 10 ppm_(v). It is noted that the permissible SMAC offormaldehyde during an EVA mission is only 0.5 ppm_(v).

The volatile organic compound may be classified as either polar ornon-polar. For purposes of this invention, the term “polar” refers to achemical compound having a dipole moment of at least about (≥0.8 DebyeD); whereas the term “non-polar” refers to a chemical compound having aweak dipole moment or no dipole moment, specifically, a dipole momentless than 0.8 D including as low as 0 D. As known in the art, dipolemoment is a measure of electrical polarity of a system of electricalcharges. Atoms that provide a dipole moment to a volatile organiccompound include, but are not limited to, oxygen, nitrogen, halogen, andsulfur. Suitable non-limiting examples of oxygen-containing substituentsthat impart a dipole moment to the VOC include hydroxyl, epoxy, acyl,keto, and carboxyl. Suitable non-limiting examples ofnitrogen-containing substituents that impart a dipole moment includeamine and amide. Suitable non-limiting examples of halogen-containingsubstituents that impart a dipole moment include fluorine, chlorine,bromine, and iodine; and suitable non-limiting examples ofsulfur-containing substituents that impart a dipole moment includethiol, sulfite, sulfate, and thionyl. Purely organic substituentsconsisting of hydrogen and carbon atoms can also provide a dipole momentto the volatile organic compound depending upon position(s) and numberof organic substituent(s), such organic substituents including but notlimited to methyl, ethyl, propyl, and higher homologues thereof.

In one exemplary embodiment, the volatile organic compound is a polarcompound having a dipole moment of at least about 1.5 D. In anotherexemplary embodiment, the volatile organic compound is a polar compoundhaving a dipole moment of at least about 2.0 D. In yet another exemplaryembodiment, the volatile organic compound is a polar compound having adipole moment of at least about 2.5 D. At the upper limit the polar VOCtypically has a dipole moment less than about 15 D.

The volatile organic compound in one exemplary embodiment comprises anodoriferous compound or an irritant, for example, an irritant towardsskin and/or eyes. In another embodiment the volatile organic compoundcomprises a pollutant or contaminant, which we define as a chemicalcompound that is classified as noxious, hazardous or otherwise harmfulto humans in a concentration greater than an established thresholdlevel. Reference is made herein to the “Toxic and Hazardous Substances”List, Table Z-1, of the Occupational Safety and Health Standards,distributed by the Occupational Safety and Health Administration (OSHA),where the skilled person finds a list of contaminants and pollutants,many of them classifying as polar VOC's, along with their maximumallowable concentration in air. Reference is also made to the “PriorityPollutant List” distributed by the Environmental Protection Agency ofthe United States, wherein over 126 pollutants are identified. Amongthese lists are found various non-limiting examples of VOC's includingacetaldehyde, acetic acid, acetone, acetonitrile, acrolein, acrylamide,acrylonitrile, allyl alcohol, allyl chloride, aminoethanol, aniline,benzyl chloride, butane thiol, butyl alcohol, butyl amine,chloroacetaldehyde, chlorobenzene, chloroform, cyclohexanol,dichlorobenzene, dichloromethane, dimethylamine, dihydroxymethane,dioxane, ethanol, ethanethiol, ethyl acetate, ethylamine, formaldehyde,formic acid, furan, methanol, methyl mercaptan, methyl acetate, methylacrylate, methyl bromide, methyl ethyl ketone, phenol, propylene oxide,tetrahydrofuran, and vinyl chloride. It should be appreciated thatcertain VOC's may be classified into several of the aforementionedcategories; for example, an odoriferous VOC or irritant may also beclassified as a pollutant or hazardous material. Additionally, it shouldbe appreciated that in another embodiment the gaseous feedstreamcomprises a mixture of such VOC's.

In another exemplary embodiment the volatile organic compound isselected from the group consisting of C₁₋₈ oxy-substituted hydrocarbonsand C₁₋₈ halocarbons and mixtures thereof. Preferred non-limitingexamples of C₁₋₈ oxy-substituted hydrocarbons include C₁₋₈ aldehydes,ketones, epoxides, alcohols, carboxylic acids, and mixtures of theaforementioned compounds having from 1 to 8 carbon atoms. In anotherpreferred embodiment, the volatile organic compound is a C₁₋₈ aldehydeor a mixture of C₁₋₈ aldehydes, illustrative species of which includeformaldehyde, propionaldehyde, and butyraldehyde. In yet anotherpreferred embodiment, the volatile organic compound is formaldehyde.Suitable non-limiting examples of C₁₋₈ halocarbons include C₁₋₈chlorocarbons, such as carbon tetrachloride, C₁₋₈ hydrochlorocarbons,such as methylene dichloride, and C₁₋₈ fluorochlorocarbons, such asfluorotrichloromethane.

In addition to ammonia and any VOC, the gaseous feedstream comprises oneor more other gases, these including chemical compounds that are notharmful to humans and chemical compounds that may be harmful but do notqualify as a VOC. In one embodiment, the other gases in the feedstreaminclude at least one naturally occurring gas including but not limitedto molecular oxygen, nitrogen, water, carbon dioxide, noble gases(helium, neon, argon, krypton, xenon), or any mixture thereof as found,for example, in air. In another embodiment, the other gases in thefeedstream include waste gases produced by combustion, which comprisewater, carbon monoxide, carbon dioxide, or any mixture thereof.

The relative humidity of the gaseous feedstream fed to the processes ofthis invention ranges from 0 percent to less than about 80 percent.“Relative humidity”, expressed as a percentage, refers to an amount ofwater vapor present in the feedstream at a given temperature andpressure as compared with a maximum amount of water vapor that thefeedstream can hold at the same temperature and pressure, i.e., thesaturation amount. In one preferred embodiment, the relative humidity ofthe gaseous feedstream ranges from about 0 percent to about 40 percent.

We have discovered that the graphene-based sorbents of this inventionare essentially non-responsive to carbon dioxide and further that carbondioxide has essentially no adverse effects on these sorbents.Accordingly, the gaseous feedstream in one embodiment comprises anyconcentration of carbon dioxide less than 100 volume percent.Consequently, the sorbents of this invention beneficially provide forfull use of their capacity towards sorbing ammonia and any VOC withoutundesirable loss of capacity towards carbon dioxide sorption.

The novel sorbents of this invention for sorbing ammonia and thevolatile organic compound employ graphene-based nano-materialscharacterized by a high surface area. It should be appreciated thatgraphene comprises a 2-dimensional crystalline allotrope of carbon inwhich carbon atoms are densely packed in a regular array of sp²-bonded,atomic scale hexagonal pattern. Graphene can be described as a one-atomthick layer or sheet of graphite, as disclosed by H. Schniepp et. al. in“Functionalized Single Graphene Sheets Derived from Splitting GraphiteOxide,” The Journal of Physical Chemistry B, Vol. 110, 17, 2006,8535-8539. Graphene functionalized with oxygen-bearing substituents isfrequently referred to as “graphene oxide,” which likewise comprises a2-dimensional crystalline allotrope of carbon in which carbon atoms aredensely packed in a regular array of sp²-bonded, atomic scale hexagonalpattern. Graphene oxide, however, further comprises epoxy (/^(O)\) andhydroxyl (—OH) groups (or substituents) bonded to the surface of thegraphene sheet as well as carboxyl [—C(O)OH] and hydroxyl (—OH) groupsbonded to the edges of the sheet.

Generally, the oxygen in graphene oxide occurs as a mixture of hydroxyl,epoxy, and carboxyl substituents and is present in a concentrationgreater than about 5 percent, preferably, greater than about 10 percent,by weight, based on the weight of the graphene oxide. Generally, theoxygen is present in a concentration less than about 50 percent,preferably, less than about 40 percent, by weight, based on the weightof the graphene oxide. Of these, the carboxyl functionality representsfrom 30 to 100 percent by weight of the total oxygen, depending upon howthe graphene oxide is prepared. In the [N]- and [O]-functionalizedgraphene sorbent of this invention, a proportion of oxygen substituentsconverted to nitrogen-containing substituents is typically greater than5 percent, preferably greater than about 15 percent, and morepreferably, greater than about 25 percent, up to essentially 100percent.

More specifically, the novel graphene-based sorbent for adsorbingammonia comprises graphene oxide having supported thereon at least onecompound selected from metal salts, metal oxides, and acids, each ofwhich is capable of reversibly adsorbing ammonia. The phrase “supportedthereon” is non-limiting, referring broadly to any method of bonding theat least one compound selected from metal salts, metal oxides, and acidsto the graphene oxide, as well as referring to any resulting bindingconfiguration including physisorption, chemisorption, ionic bonding, Vander Waals bonding, covalent bonding, complexing, and the like. Suchmetal salts are comprised of a positively-charged metallic cation and anegatively-charged anion. Non-limiting examples of suitablepositively-charged metallic cations include at least one of the cationsselected from the metals of Groups 1 and 2, and the first row oftransition elements of Groups 3 through 12 of the Periodic Table of theElements (International Union of Pure and Applied Chemistry, Nov. 28,2016). As used herein, the Group 1 metals include lithium, sodium,potassium, rubidium, and cesium. The Group 2 metals include beryllium,magnesium, calcium, strontium, and barium. The first row transitionelements of Groups 3 through 12 include scandium, titanium, vanadium,chromium, manganese, iron, cobalt, nickel, copper, and zinc. Preferredfor purposes of this invention are the cations of lithium, magnesium,calcium, strontium, iron, copper, zinc, and mixtures thereof. Thenegatively-charged anion of the metal salt may be any common anionincluding at least one of a halide, sulfate, nitrate, or phosphate. Inone embodiment, the anion is a halide selected from fluoride, chloride,bromide, iodide, and mixtures thereof. Accordingly, among suitablenon-limiting embodiments, the metal salt capable of reversibly sorbingammonia is selected from the group consisting of lithium chloride,magnesium chloride, calcium chloride, strontium fluoride, iron chloride,copper chloride, zinc chloride, and mixtures thereof.

Suitable metal oxides that are capable of reversibly sorbing ammonia areselected from the oxides of the metals of Groups 1 and 2, and the firstrow of transition elements of Groups 3 through 12 of the Periodic Tableof the Elements (International Union of Pure and Applied Chemistry, Nov.28, 2016), such elements as described hereinbefore. Preferred forpurposes of this invention are the oxides of lithium, magnesium,calcium, strontium, iron, copper, zinc, and mixtures of such oxides.Suitable acids that reversibly sorb ammonia include, without limitation,hydrochloric acid, phosphoric acid, nitric acid, and sulfuric acid, withphosphoric acid being a preferred embodiment.

A general method of preparing the ammonia sorbent of this inventioninvolves contacting graphene oxide with at least one of theaforementioned metal salts, metal oxides, acids, or mixture thereofcapable of reversibly adsorbing ammonia, the contacting including any ofimpregnating, immersing, spraying, sonicating, complexing, or otherwisechemically reacting graphene oxide with one or more solutions orslurries containing the metal salt(s), oxides(s), acid(s), or mixturethereof. It should be appreciated that graphene oxide can be purchased(e.g., Angston Corp.), but can also be prepared by ultrasonicatinggraphite oxide, or alternatively prepared by oxidizing graphite with amixture of strong oxidants, such as sulfuric acid and potassiumpermanganate, followed by rapid heating of the resultant oxidizedgraphite under an inert atmosphere. After contacting the graphene oxidewith the metal salt(s), metal oxide(s), and/or acid(s), the resultingcomposite is dried under air, typically, at a temperature ranging fromabout 50° C. to about 120° C. Generally, the metal salt, metal oxide,acid or mixture thereof is loaded onto the graphene oxide in a totalamount ranging from about 0.05 weight percent to about 40 weightpercent, based on the weight of the graphene oxide.

The graphene-based sorbent for sorbing the volatile organic compound isprepared generally by contacting graphene oxide with an amine of theformula NHR₂, wherein each R is independently selected from the groupconsisting of hydrogen, C₁₋₇ alkyl, and C₁₋₇ aminoalkyl, the contactingoccurring under process conditions sufficient to prepare the nitrogenand oxygen-functionalized graphene. In one embodiment, the amine isrepresented by formula NHR₂, wherein each R is independently selectedfrom the group consisting of hydrogen, C₁₋₅ alkyl, and C₁₋₅ aminoalkyl;illustrative non-limiting examples of which include methylamine,ethylamine, propylamine, dimethylamine, diethylamine, and dipropylamine.In another embodiment, each R is hydrogen and the amine is ammonia,suitably provided in an aqueous solution as ammonium hydroxide. Morespecifically, graphene oxide is solubilized or suspended in a suitablesolvent and reacted with the amine of the formula NHR₂ at a temperaturesufficient to promote the appropriate substitution or thermochemicalreaction of the amine with the oxygen functionalities on the grapheneoxide. In one preferred synthesis, graphene oxide is reacted withaqueous ammonia (ammonium hydroxide) at a temperature ranging from about20° C. to about 110° C. for a time ranging from about 5 hours to about48 hours. The quantity of ammonium hydroxide employed is sufficient toconvert at least 10 percent and up to 100 percent of the oxygenfunctionalities to nitrogen functionalities. In another embodiment anexcess of ammonium hydroxide relative to oxygen functionalities isemployed. Following the thermal treatment, the solution is filtered andthe resulting nitrogen and oxygen-functionalized graphene is thoroughlywashed with water, then dried at a temperature ranging from about 70° C.to 120° C. to obtain the solid nitrogen and oxygen-functionalizedsorbent of this invention. Where the amine (NHR₂) is an alkylamine oralkyldiamine, a suitable solvent or diluent, such as water or C₁₋₃alcohol, is employed; or the amine itself acts as the solvent; and thereaction conditions are closely similar to those mentioned above as aperson skilled in the art will appreciate.

We believe, although such belief is theory and should not be limiting inany manner, that the amine (NHR₂) reacts with the hydroxyl and carboxylsubstituents on the graphene oxide giving rise, respectively, to amine(—NR₂) and amide [—C(O)NR₂] functionalities bonded to the graphene.Additionally, the amine may react with the epoxy substituents on thegraphene oxide giving rise to both hydroxyl (—OH) and amine (—NR)functionalities, although the epoxy substituents are considered to beless reactive than the hydroxyl and carboxyl substituents. Accordingly,the sorbent of this invention is believed to comprise graphenefunctionalized with a plurality of two types of substituents: (a) anoxygen-containing substituent selected from the group consisting ofhydroxyl (—OH), epoxy (/^(O)\) and carboxyl [—C(O)OH], and mixturesthereof; and (b) a nitrogen-containing substituent selected from thegroup consisting of amine (—NR₂), amide [—C(O)NR₂], and mixturesthereof, wherein each R again is independently selected from hydrogen,C₁₋₇ alkyl, and C₁₋₇ aminoalkyl. It is however possible that the sorbentalso comprises ionic bonded amine in the form of quaternary ammoniumcarboxylates represented by [—C(O)O⁻⁺HNHR₂]. Note that even in theinstance wherein all hydroxyl, carboxyl, and epoxy groups have reactedwith amine, the sorbent product is likely to contain amine, amide, andhydroxyl groups providing for both nitrogen andoxygen-functionalization.

Generally, the graphene sorbents of this invention have a particle sizecorrelating substantially to the particle size of the unmodifiedgraphene oxide from which the sorbent is derived. Since the sorbent andthe unmodified graphene oxide are both essentially two-dimensionalmaterials, the thickness of the particles is significantly smaller thanthe width of the particles. In typical embodiments, the thickness of thesorbent particles ranges from about 1 nanometer (1 nm) to less thanabout 50 nm, as determined by transmission electron microscopy (TEM) orscanning electron microscopy (SEM). The width of the particles rangesfrom greater than about 100 nm, preferably, greater than about 200 nm,to less than about 10 microns (μm). Various conventional methods, suchas ball-milling, sonication, and thermal annealing, can be employed tomodify the size of the particles and/or to select a range of desiredparticle sizes. The sorbents of this invention generally exhibit a highsurface area similar to the surface area of the graphene oxide startingmaterial. A typical surface area ranges from about 50 m²/g to about1,000 m²/g, and in one embodiment, between about 200 m²/g and about 500m²/g.

The sorbents of this invention are provided to the adsorption processesin any of a variety of physical forms including but not limited topowders, pellets, extrudates, or alternatively, as a layer, laminate orcoating coupled to a non-porous or macroporous support, such supports toinclude ceramic and metallic fibers, meshes, foams, and monoliths. Theterm “macroporous” refers to pores, channels, or void spaces having acritical diameter or dimension larger than about 0.5 micron (>0.5 μm),and preferably, larger than about 25 μm. In one embodiment, the sorbentis provided as a layer or a coating covering a support in the form of anon-porous wall. In another embodiment, the sorbent is provided as alayer or a coating covering a macroporous support in the form of pelletsor extrudates. In another embodiment, the sorbent is applied to a highsurface area support, such as a support having a surface area of atleast about 100 m²/g for the purpose of increasing access of the gaseousfeedstream to the sorbent as well as decreasing pressure drop across thesorbent bed. In yet another embodiment, the sorbent further comprises abinder, which functions to impart an acceptable degree of cohesivenessand attrition resistance to the sorbent. Supports and binders forsorbents are found in the art.

In a preferred embodiment, the ammonia and VOC sorbents are eachindividually applied as a layer or coating to a high surface area meshsupport, preferably, a Microlith® brand mesh support having anultra-short-channel-length (Precision Combustion, Inc., North Haven,Conn.). The mesh support is in the form of a reticulated net or screen,that is, a substantially two-dimensional lattice wherein a thicknessdimension is substantially smaller than length and width dimensions, andwherein the lattice contains a regular or irregular array of short poresand channels. In terms of materials of construction, the mesh support isselected from metal meshes, ceramic meshes, cermet meshes, andcombinations thereof. The mesh is not limited by any method ofmanufacture; for example, meshes can be constructed via weaving orwelding fibers, or by an expanded metal technique as disclosed in U.S.Pat. No. 6,156,444, incorporated herein by reference, or by 3-Dprinting, or by a lost polymer skeleton method.

In more specific embodiments, the metal mesh support is constructed fromany thermally and electrically conductive metal or combination of suchmetals provided that the resulting structure is capable of withstandingthe temperatures and chemical environment to which the mesh is exposed.Suitable non-limiting materials for the metal mesh support includeiron-chromium alloys, iron-chromium-aluminum alloys, andiron-chromium-nickel alloys. Such metal meshes are availablecommercially, for example, from Alpha Aesar and Petro Wire & Steel. Inone embodiment, the metal mesh comprises a Microlith® brand metal meshobtainable from Precision Combustion, Inc., of North Haven, Conn., USA.

Pertaining to ceramic meshes, the term “ceramic” refers to inorganicnon-metallic solid materials with a prevalent covalent bond, includingbut not limited to metallic oxides, such as oxides of aluminum, silicon,magnesium, zirconium, titanium, niobium, and chromium, as well aszeolites and titanates. Reference is made to U.S. Pat. Nos. 6,328,936and 7,141,092, detailing insulating layers of short channel ceramic meshcomprising woven silica, both patents incorporated herein by reference.Pertaining to cermet meshes, the term “cermet” refers to a compositematerial comprising a ceramic in combination with a metal, the compositebeing typically conductive while also exhibiting a high resistance totemperature, corrosion, and abrasion in a manner similar to ceramicmaterials.

More specifically, the mesh support is configured typically with aplurality of channels or pores having a diameter ranging from about 0.25millimeters (mm) to about 1.0 mm, with a void space greater than about60 percent, preferably up to about 80 percent or more. A ratio ofchannel length to diameter is generally less than about 2:1, preferablyless than about 1:1, and more preferably, less than about 0.5:1.Preferably, the mesh has a cell density ranging from about 100 to about1,000 cells or flow paths per square centimeter.

As described in U.S. Pat. Nos. 5,051,241 and 6,156,444, incorporatedherein by reference, Microlith® brand mesh technology offers a uniquedesign combining an ultra-short-channel-length with low thermal mass inone monolith, which contrasts with prior art monoliths havingsubstantially longer channel lengths as noted hereinabove. For thepurposes of this invention, the term “ultra-short-channel-length” refersto a channel length in a range from about 25 microns (μm) (0.001 inch)to about 500 μm (0.02 inch). In contrast, the term “long channels”pertaining to prior art monoliths refers to channel lengths greater thanabout 5 mm (0.20 inch) upwards of 127 mm (5 inches).

The loading of the graphene-based sorbent onto the mesh support isdescribed in units of weight sorbent per unit volume of support; andthis advantageously ranges in one embodiment from about 50 mg sorbentper cubic centimeter support (50 mg/cm³) to about 1,500 mg/cm³. Inanother embodiment, the loading ranges from about 100 mg/cm³ to about750 mg/cm³. This description takes gross dimensions of the support intoaccount. The thickness and uniformity of the sorbent coating on thesupport vary depending upon the specific support, sorbent, and coatingmethod selected.

It should be appreciated that in one exemplary embodiment, the sorbentcompositions of this invention exclude any macroporous and mesoporousco-sorbent, including any co-sorbent carbon nanotube (CNT), zeolite,molecular sieve, activated carbon, or mixture thereof. Such co-sorbentstypically contain a regular or irregular system of tubes, pores,channels, or void spaces having a critical dimension ranging from about0.5 nanometer (0.5 nm) to about 50 nm, which we find undesirable for tworeasons. Firstly, such microporous and mesoporous co-sorbents arelimited to trapping only VOC's that fit within their tubular or poresystem, that is, those VOC's with dimensions smaller than the dimensionsof the tubes, pores, channels, or void spaces. Secondly, VOC's thatenter the tubular or pore system may find it difficult to exit. As aconsequence, regenerating microporous and mesoporous co-sorbents isdifficult resulting in undesirable loss in sorbent capacity.Additionally, CNT's in particular are difficult to fabricate in largescale thereby adding unnecessary costs to manufacture. In contrast, ourgraphene-based sorbents are each prepared in one simple, cost effectivestep; are essentially non-porous (i.e., essentially do not contain poresand channels), and exhibit excellent regenerability.

In one embodiment, the process of removing ammonia from a gaseous streamis conducted in a single sorbent bed, wherein in adsorption mode a flowof gaseous feedstream containing ammonia in an initial concentration iscontacted with a fixed bed of the ammonia sorbent for a time duringwhich an effluent stream exiting the sorbent bed contains an acceptablyreduced concentration of ammonia. When the ammonia sorbent bed is fullyor partially saturated and the effluent stream contains an unacceptableconcentration of ammonia (a condition known as “break-through”), theflow of gaseous feedstream containing ammonia is stopped. Thereafter,the sorbent is regenerated by running the bed in desorption mode by oneor a combination of methods including: heating the sorbent bed, orexposing the sorbent to a pressure gradient, namely, a decreasedpressure or vacuum; or passing a sweep gas through the sorbent bed todrive off the adsorbed ammonia, which is typically collected in acontainment vessel or exhausted into an exterior atmosphere. Suitablesweep gases include air, nitrogen, carbon dioxide, helium, argon, andthe like, with air being a preferred sweep gas; any of these beingprovided in “clean” form, that is, essentially absent absorbablecontaminants. In a preferred embodiment, the ammonia sorbent bed isregenerated by exposing the bed to a pressure gradient, that is, adecreased pressure or a vacuum of less than about 1 Torr, at ambienttemperature, taken as about 21-25° C., although it may vary depending onthe ambient condition. Thereafter, the process involves alternating thesorbent bed between adsorption and desorption modes over manyreiterations, pressure swing regeneration being preferred.

In terms of operation, in one embodiment, the second process of removingammonia and a volatile organic compound from a gaseous feedstream iseffected in a staged reactor wherein a first bed comprises the ammoniasorbent and a second bed comprises the formaldehyde sorbent; or viceversa as desired. In one embodiment, the first and second beds areprovided within one housing. In another embodiment, the first and secondbeds are provided in separate housings. Preferably, the ammonia sorbentis disposed upstream of the VOC sorbent. In the aforementionedembodiments, the sorbents can be provided in powder form or provided ona support; in a preferred embodiment supported on the Microlith® brandmesh support described hereinbefore. In the latter embodiment, aplurality of mesh-supported sorbents are stacked either sequentially orin alternating layers or in any other desired configuration within oneor more housings.

In adsorption mode for removing multiple contaminants, a flow of gaseousfeedstream containing ammonia and VOC sorbates in their respectiveinitial concentrations is contacted with the combined sorbent system soas to remove the ammonia and the VOC, until such time as one or both ofthe sorbates breaks through the effluent stream in an unacceptableconcentration. At such time, the flow of gaseous feedstream to thesorbent bed(s) is stopped; and thereafter the sorbents are regenerated.Regeneration (or desorption mode) includes one or more of heating thesorbent beds, or decreasing pressure, e.g., pulling a vacuum on thesorbent beds, or passing a sweep gas through the sorbent beds, such thatdesorbed ammonia and the VOC are either collected in a containmentvessel or exhausted into an exterior atmosphere. Suitable sweep gasesinclude air, nitrogen, carbon dioxide, helium, argon, and the like, withair being a preferred sweep gas; notably, the sweep gas is “clean,”essentially excluding absorbable contaminants. A preferred regenerationmethod involves exposing the sorbent system to a pressure gradient, forexample, by pulling a vacuum (≤1 Torr) on the system at ambienttemperature, taken as about 21-25° C., although it may vary depending onthe ambient condition. Thereafter, the process involves alternating thesorbent system between adsorption and desorption modes over manyiterations.

In another embodiment of the aforementioned processes, a plurality ofsorbent beds is engaged in swing mode operation such that one or moresorbent beds are operated in adsorption mode, while one or more othersorbent beds are simultaneously operated in desorption mode. As the bedsoperating in adsorption mode reach a desired partial or full saturation,the bed operations are switched such that the bed(s) originallyoperating in adsorption mode are engaged in desorption mode, while thebed(s) originally operating in desorption mode are converted toadsorption mode. Temperature swing operation involves cycling the bedsbetween a temperature suitable for effecting adsorption and atemperature, usually a higher temperature, suitable for effectingdesorption. Pressure swing operation involves cycling the beds between apressure gradient, for example, exposure to the VOC at normal orelevated pressure to effect adsorption and then exposure to a lowerpressure, such as a vacuum, to effect desorption. Swing bed technologyis known in the art and advantageous in eliminating downtime while a bedis regenerated.

Valves for directing the flow(s) into and out of each sorbent bed can beany of those commercially available flow control valves known to aperson skilled in the art. Likewise, valves for exposing each sorbentbed to a pressure gradient include any of such pressure control valvesthat are known to a skilled person and generally available commercially.The term “pressure gradient” means that the pressure control valveconnects two environments at different pressures; for example, thepressure of the contaminant in the sorbent bed when the bed is loaded istypically higher than the pressure of the contaminant in an environmentoutside the sorbent bed. Sensors detecting a concentration of thecontaminant in each sorbent bed or in an effluent stream exiting eachsorbent bed can be any commercially available sensor suitable fordetecting the contaminant of interest. Such sensors include, forexample, flame ionization detectors and thermal conductivity detectors.Finally, a controller responsive to the sensor(s) or a predeterminedtime period for controlling operation of the plurality of valves can beobtained commercially or constructed by a person skilled in the art.

The adsorption-desorption processes of this invention are conductedunder any process conditions providing for acceptable reduction ofammonia or reduction of ammonia and volatile organic compound,preferably formaldehyde, from the gaseous feedstream. The followingprocess conditions are presented for guidance purposes; however, otherprocess conditions may be operable and desirable. The ammonia or theammonia-VOC (e.g., formaldehyde) adsorption cycle is operatedadvantageously at a sorbent bed temperature ranging from about 5° C. toabout 50° C. and a pressure ranging from about 1 atm (101 kPa) to about5 atm (506 kPa). In one exemplary embodiment, the adsorption cycle isoperated at about ambient temperature and pressure, taken as 21-25° C.and about 1 atm (101 kPa). Advantageously, during the adsorption cyclethe gaseous feedstream containing ammonia or containing ammonia and VOC(formaldehyde) is fed to the sorbent bed at a gas hourly space velocityranging from about 100 ml total gas flow per ml sorbent bed per hour(hr⁻¹) to about 100,000 hr⁻¹. The desorption cycle is beneficiallyoperated by simply exposing the partially saturated sorbent(s) to alower pressure at a temperature less than about 40° C., in oneembodiment from about 21° C. to about 35° C., and preferably at ambienttemperature, taken as 21° C.-25° C. Advantageously, the desorption cycleoperates at a total pressure ranging from about 0.0002 atm (0.02 kPa) toabout 1 atm (101 kPa).

In adsorption mode, the first process of this invention achieves a lowerconcentration of ammonia in the effluent stream exiting the sorbent bed,as compared with the initial concentration of ammonia in the feedstreamto the bed. In adsorption mode, the second process of this inventionachieves lower concentrations of ammonia and the volatile organiccompound in the effluent stream exiting the sorbent bed, as comparedwith the initial concentrations of ammonia and volatile organic compoundfed to the bed. Generally for spacesuit and aerospace applications theconcentration of ammonia in the effluent stream is advantageously lessthan about 20 ppm_(v) more preferably, less than about 10 ppm_(v), basedon the total volume of the effluent stream exiting the adsorption bed.Generally, for spacesuit and aerospace applications, the concentrationof formaldehyde in the effluent stream is advantageously less than about0.6 ppm_(v) preferably, less than about 0.2 ppm_(v) based on the totalvolume of the effluent stream exiting the adsorption bed.

One property of the sorbents of this invention should be fullyappreciated as distinct from sorbents known in the art. Specifically,the sorbents of this invention are capable of regeneration under mildconditions, which means that when the sorbents are fully or partiallysaturated with sorbate(s), their adsorption capacity is regenerableunder a reduced pressure or vacuum, or under a flow of a sweep gas, suchas essentially clean air, at a temperature less than about 40° C., inone embodiment less than about 35° C., and at about 1 atm (101 kPa).This allows for the sorbents of this invention to be regenerated withoutapplication of significant heat and therefore with low powerconsumption. We have further discovered that our sorbents can be cycledthrough at least about 3 adsorption-desorption cycles without losingmore than 20 percent of their original capacity for sorbent.

Embodiments Example 1 (E-1)

An embodiment of the ammonia sorbent of this invention was prepared andevaluated in removing ammonia from a gaseous environment. A rig foradsorption and desorption testing was constructed as follows. A singlefixed sorbent bed comprising a cylindrical tube [stainless steel, 0.8inch inner dia. (2.0 cm), 0.5 inch length (L25 cm)] was fitted at eachend with a flow line and a conventional flow control valve, on theupstream end for controlling a flow of gaseous feedstream into the bedand on the downstream end for exiting an effluent flow from the bed. Thetube was provided with a voltage controller and wrapped with a heatingtape so as to provide heating to the bed. A humidity control wasconnected to the upstream flow line to provide gaseous water to thefeedstream. A commercial supply of ammonia in air (Airgas) was dilutedwith a supplemental supply of clean air (compressed air absentcontaminants) to yield the gaseous feedstream of desired NH₃ contaminantconcentration. Water was fed into the supplemental supply of air tobring the feedstream humidity to a desired level, as measured by arelative humidity detector (HMC 20, Vaisala). The concentration ofammonia in the effluent stream was analyzed by passing the effluentstream through a conventional catalytic oxidizer, which converted anyammonia in the effluent stream into nitrogen oxides (NOx), which in turnwere analyzed in a chemiluminescence detector (Thermo Scientific).

A sorbent comprising graphene oxide having supported thereon calciumchloride was prepared as follows. A solution of calcium chloride (100 mgCaCl₂ in 100 ml deionized water) was added drop-wise onto a sample ofgraphene oxide (1 g, Angstron Materials, catalog number N002-PDE;graphene oxide platelets with a thickness of 2-3 nanometers (2-3 nm);lateral dimension of approximately 7 micrometers (7 μm); carbon content60-80 percent; oxygen content between 10-30 percent; and a surface areaof 420 m²/gm.). The resulting CaCl₂-impregnated material was dried at80° C. to yield the ammonia sorbent as a powder.

Operating conditions during adsorption mode were as follows: sorbent bed(900 mg); feedstream inlet pressure, 2-3 psig (14-21 kPa); temperature,22° C.; flow rate, 1 standard liter per minute (1 slpm) contaminated aircontaining ammonia (20 ppm_(v)); relative humidity, 20 percent.Break-through was defined as the time at which the concentration ofammonia in the effluent stream equaled 50 percent of the inlet ammoniaconcentration, namely, 10 ppm_(v).

Results are shown in FIG. 1 , where breakthrough in the first cycle wasseen at 200 minutes. The sorbent bed was thereafter subjected todesorption by exposing the bed to a stream of clean dry air at ambienttemperature, that is, without heat input. Then the bed was subjected toa second adsorption cycle with the same ammonia-contaminated air.Results are shown in FIG. 1 , wherein breakthrough for the secondadsorption cycle was seen at 180 minutes. Thereafter, the bed wassubjected to a second desorption cycle under clean, dry air at roomtemperature without heat input. Then, the bed was subjected to a thirdadsorption cycle with the same stream of ammonia-contaminated air.Results are shown in FIG. 1 , wherein breakthrough for the thirdadsorption cycle was found at 190 minutes. The data show consistency inthe NH₃ outlet concentration over all three adsorption cycles, whichindicates good sorption capacity and excellent regenerability under mildconditions. The sorbent achieved an overall average cyclic capacity of2.8 mg NH₃/g sorbent.

Example 2 (E-2)

An embodiment of this invention comprising the ammonia sorbent supportedon a metal mesh support was prepared and evaluated in the removal ofammonia from a gas environment. A sorbent comprising graphene oxidehaving supported thereon calcium chloride was prepared in the mannerdescribed in Example 1. The sorbent was applied with a binder to aMicrolith®-brand ultra-short-channel-length metal mesh (PrecisionCombustion, Inc.) having a channel length of 250 microns.

The metal mesh (100 sheets) supporting the ammonia sorbent was placed ina cylindrical fixed bed test rig (2.0 cm inner dia., 0.8 inch),otherwise constructed as in Example E-1. The mesh-supported sorbent wasevaluated for ammonia sorption capacity in a manner similar to E-1.Operating conditions during adsorption mode were as follows: inletpressure of the feedstream to the sorbent bed, 2 Pa gage pressure(101.35 kPa absolute); temperature of the sorbent bed, ambient (22° C.);flow rate, 1 slpm contaminated air containing ammonia (20 ppm_(v));relative humidity, 20 percent. Break-through was taken at 5 ppm_(v)ammonia in the outlet stream, at which time adsorption was stopped. Thepressure drop measured across the metal mesh sorbent bed was less than25 Pa, which was significantly lower than that measured across thepowder bed of Example E-1. Regeneration was conducted under vacuum (300mTorr, 40 Pa absolute) without application of heat. The bed wassubjected to a total of three adsorption cycles with interveningdesorption cycles with the results shown in FIG. 2 . As observed, fullammonia sorption was observed for 18 minutes in each cycle withsuccessful regenerability. The bed was subjected to 17 additionaladsorption cycles with intervening desorption cycles with no observabledeterioration in sorbent capacity or regeneration capability.

Example 3 (E-3)

An embodiment of the process and sorbent system of this invention wasevaluated in removing ammonia and formaldehyde from air containing thesetrace contaminants. The experimental setup of Example E-2 was employed,with the exception that the sorbent bed consisted of a combination ofthe graphene-based ammonia sorbent of Example E-2 and the functionalizedgraphene-based formaldehyde sorbent of this invention, each supportedindependently on the Microlith®-brand metal mesh employed in Example 2.The ammonia sorbent was placed upstream of the formaldehyde sorbent inthe test rig.

The formaldehyde sorbent (240 mg) comprising the nitrogen andoxygen-functionalized graphene was prepared by thermal treating acommercial graphene oxide with aqueous ammonium hydroxide. The grapheneoxide in powder form (Angstron Materials, catalog number N002-PDE)comprised a few-layer graphene oxide platelets with a thickness of 2-3nanometers (2-3 nm); a lateral dimension of approximately 7 micrometers(7 μm); a carbon content of 60-80 percent; oxygen content between 10-30percent; and a surface area of 420 m²/gm. The powder was mixed in aflask with excess aqueous ammonium hydroxide (30 wt. percent solution)in a ratio of 30 g ammonium hydroxide solution per gram graphene oxide.The mixture was heated to 90-100° C. under reflux for 48 h.Periodically, the level of the mixture was checked and replenished asneeded, as some ammonia gas was released from solution under reactionconditions. At the end of the 48 h a solid product was recovered byfiltration and washed with deionized water until the pH of the filtratewas measured at neutral. The solid was further dried at 70° C. overnightto yield the formaldehyde sorbent as a powder. The sorbent was appliedwith a binder to the Microlith®-brand metal mesh (Precision Combustion,Inc.) of Example 2 yielding the mesh-supported formaldehyde sorbent ofthis invention.

The two graphene-based sorbents were positioned within the cylindricalfixed bed, the ammonia sorbent (85 weight percent) placed upstream ofthe formaldehyde sorbent (15 weight percent). The combination sorbentbed was exposed to a feedstream comprising ammonia (20 ppm_(v)) andformaldehyde (5 ppm_(v)), a relative humidity of 20 percent, balancebeing air. Total flow through the composite sorbent bed was 1 slpm at22° C. and 1 atm (101 kPa). Regeneration was performed by applying avacuum at ambient temperature (22° C.) without added heat. Fouradsorption cycles were run, with three intervening desorption cycles.

FIG. 3 depicts a graph of ammonia sorption in the outlet stream of theprocess as a function of time. Throughout the test, the outletconcentration of formaldehyde was measured at less than 0.1 ppm_(v). Allfour adsorption cycles showed consistent profiles. Full ammonia removalwas achieved for about 15 minutes in each run, while formaldehyde wasfully removed throughout the tests. The pressure drop across the sorbentbed was measured at less than 25 Pa. The results demonstrate efficientremoval of ammonia and formaldehyde with proven vacuum (pressure swing)regenerability.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions, or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

The invention claimed is:
 1. A sorbent system comprising the followingcomponents: (a) graphene oxide having supported thereon at least onecompound selected from metal salts, metal oxides and acids, each ofwhich is capable of reversibly adsorbing ammonia; and (b) a nitrogen andoxygen-functionalized graphene prepared by a process comprising,contacting graphene oxide with an amine of the formula NHR₂, whereineach R is independently selected from the group consisting of hydrogen,C₁₋₇ alkyl, and C₁₋₇ aminoalkyl, the contacting occurring under reactionconditions sufficient to produce the nitrogen- and oxygen-functionalizedgraphene.
 2. The sorbent system of claim 1 wherein the metal of themetal salts and metal oxides is selected from the group consisting ofcations of Groups 1, 2, first row transition metals of Groups 3 through12 of the Periodic Table, and mixtures thereof; and wherein the acidsare selected from the group consisting of hydrochloric acid, sulfuricacid, and phosphoric acid.
 3. The sorbent system of claim 1 wherein themetal of the metal salts and metal oxides is selected from the groupconsisting of cations of lithium, magnesium, calcium, strontium, iron,copper, zinc, and mixtures thereof; and further wherein the metal saltcomprises an anion selected from the group consisting of fluoride,chloride, bromide, iodide, and mixtures thereof.
 4. The sorbent systemof claim 1 wherein the nitrogen and oxygen-functionalized graphene of(b) is prepared by contacting graphene oxide with ammonium hydroxide. 5.The sorbent system of claim 1 wherein the graphene oxide of (a) and thenitrogen and oxygen-functionalized graphene of (b) are eachindependently supported on a mesh support selected from the groupconsisting of metal, ceramic, and cermet meshes having anultra-short-channel-length ranging from 25 microns to 500 microns. 6.The sorbent system of claim 1 wherein each the graphene oxide of (a) andthe nitrogen and oxygen-functionalized graphene of (b) has a surfacearea ranging from 50 m²/g to 1,000 m²/g.
 7. The sorbent system of claim1 wherein any one or both the graphene oxide of (a) and the nitrogen andoxygen-functionalized graphene of (b) are supported on a macroporoussupport in a loading ranging from 50 mg/cm³ to 1,500 mg/cm³.